Understanding the interaction of gold and silver nanoparticles with natural organic matter using affinity capillary electrophoresis

Nanoparticles (NPs) undergo a number of changes in environmental systems which are often influenced by their interaction with natural organic matter (NOM). However, despite their importance and prevalence, these interactions are still not fully understood. To better understand the interaction between NOM and NPs, an affinity capillary electrophoresis (ACE) method is presented, which quantitatively compares the binding between various NPs and NOM by determining the binding or dissociation constant (K_D) in addition to the Hill constant (n). The Hill isotherm provided greater agreement with experimental observation than either the Langmuir or Freundlich isotherms. The method was applied to citrate stabilized gold and silver nanoparticles from different suppliers, varying in size, zeta potential and stabilizing agent content. The total amount of stabilizing agent (citrate) was measured by ion chromatography (IC) after dissolution of the NPs and the surface coverage of citrate on the NPs was determined. The surface coverage revealed that the citrate multilayer around a NP is the same regardless of the concentration of citrate in the suspension. The analysis of different NPs demonstrated that when the diameter of the NPs was less than 10 nm (surface area > 10 m² g⁻¹) the K_D was lower than that for the larger sized NPs. Additionally the citrate content in the suspension was observed to be the main chemical property influencing the binding of NOM to citrate stabilized gold and silver NPs. The proposed method can be applied to other NPs and with different testing conditions to assess how each individual chemical property of the NP and NOM influences their interaction. As such, this method will allow for better fate predictions for NPs in different environmental systems.